Field of the Invention
This invention relates to a method for the continuous dehydration of tertiary butyl alcohol, TBA, using an acid catalyst. Water formed in the dehydration reaction is continuously removed from the reaction zone as an azeotrope with xylene.
The dehydration of tertiary butyl alcohol to form isobutylene is well known.
The reaction is a relatively simple one, high temperature alone will convert tertiary butyl alcohol to isobutylene though it is usually preferred to use a catalyst.
Catalysts which have been used in the past have ranged from none, to alumina, to sulfuric acid and resin sulfonic acids.
U.S. Pat. No. 3,510,538 (U.S. Class 260-682), the teachings of which are incorporated by reference, discloses a continuous process for dehydration of tertiary butyl alcohol which uses as an acid acting catalyst a cation exchange resin, preferably a sulfonic acid type cation exchange resin.
This reference also discloses the use of benzene in the dehydration zone. The benzene forms an azeotrope with water produced by dehydration of tertiary butyl alcohol, TBA. The benzene-water azeotrope and produced isobutylene are removed as a vapor fraction from the dehydration zone. The vapors are cooled sufficiently to condense benzene and water, but not enough to condense isobutylene. A water phase is withdrawn from the process, benzene is recycled to the dehydration zone, and isobutylene product recovered as a vapor fraction.
The benzene removes water by azeotrope formation as the water is formed in the dehydration zone. This makes the dehydration zone essentially anhydrous, which promotes very high dehydration rates.
The patentee taught that reaction temperatures of 68.degree. to 100.degree. C. could be used, but that temperatures above 100.degree. C. should be avoided because isobutylene begins to dimerize appreciably at the higher temperatures. The patentee taught that although other hydrocarbons form azeotropes with water, benzene was the only compound suitable for use.
The process described is reasonably satisfactory, but not very efficient in handling TBA feed streams which contain a significant amount of water. The use of cation exchange resins as catalyst also is not desirable in that three phases are necessarily present in the reaction zone, solid, liquid, and gaseous. Presence of solid catalyst necessarily introduces problems of catalyst containment, and catalyst attrition. These problems are not impossible to overcome but do increase the complexity and cost of a dehydration reaction zone.
Finally, it would be very desirable if somewhat higher rates of reaction could be obtained. The system described in U.S. Pat. No. 3,510,538 gives relatively low conversion rates of tertiary butyl alcohol because of problems in ensuring that the liquid reactants adequately contact the particulate catalyst, and also because of the relatively low reaction zone temperatures which must be used to minimize dimerization.
Use of aryl sulfonic acids for dehydration is well known. Use of PTSA for hydration of isobutylene is also known.